L-lleu-Gly- L-Ala- D-Leu- L-Ala- D-Val- L-Val- D-Val- L-Trp- D-Leu- L-Phe- D-Leu- L-Trp- D-Leu- L-Trp-NH

Isoleucine - qramicidin B

Tyrothricin is a mixture of two groups of antibiotic compounds, the gramicidins and the tyrocidines. Gramicidins are the more active components of tyrothricin, and this fraction, which is 10% to 20% of the mixture, may be separated and used in topical preparations for the antibiotic effect. Five gramicidins, A2, A3, B1, B2, and C, have been identified. Their structures have been proposed and confirmed through synthesis by Sarges and Witkop.230 The gramicidins A differ from the gramicidins B by having a tryptophan moiety substituted by an l-phenylalanine moiety. In gramicidin C, a tyrosine moiety substitutes for a tryptophan moiety. In both of the gramicidin A and B pairs, the only difference is the amino acid located at the end of the chain, which has the neutral formyl group on it. If that amino acid is valine, the compound is either valine-gramicidin A or valine-gramicidin B. If that amino acid is isoleucine, the compound is isoleucine-gramicidin, either A or B.

Tyrocidine is a mixture of tyrocidines A, B, C, and D, whose structures have been determined by Craig et al.258,259 The synthesis of tyrocidine A was reported by Ohno et al.260

L-Val —L-Om —►

L-Leu —

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